Take a look at the source code perldoc -m Pod::Usage:

sub pod2usage {
    local($_) = shift;
    my %opts;

# ...

    ## Now determine default -exitval and -verbose values to use
    if ((! defined $opts{'-exitval'}) && (! defined $opts{'-verbose'})
+) {
        $opts{'-exitval'} = 2;
        $opts{'-verbose'} = 0;
    }
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-verbose is used verbatim as a hash key.